This invention relates generally to semiconductor device packages and packaging methods and more particularly to microwave field effect transistor (FET) packages and packaging methods.
As is known in the art, it is frequently necessary to provide a hermetically sealed package for a semiconductor device to protect such devices from external environmental effects. When such semiconductor device is to operate at microwave frequencies, as when the device is a microwave field effect transistor, the construction and configuration of the package is sometimes critical in order to minimize any effect of the package on the electrical operation or electrical characteristics of the field effect transistor. One prior art microwave field effect transistor package 10 is shown in FIG. 1 to include a gold plated copper bottom portion 12 having a lower portion 14 providing the base of such bottom portion 12. Integrally formed with the lower portion 14 is middle portion 16. Disposed on and bonded to the right and left upper surface of the middle portion 16 is a pair of strip transmission lines 18, 20 each one having a dielectric substrate 22, 23 with a ground plane conductor 24, 25 on the bottom surface thereof bonded to the upper surface of the middle portion 16 and strip conductors 26, 27 formed on the upper surfaces of the substrates 22, 23. A field effect transistor 28 to be packaged has the source electrode contact 29 thereof formed on the bottom surface of the transistor 28 and bonded to the upper surface of upper portion 30 of bottom portion 12, such upper portion 30 being integrally formed with middle portion 16. The gate electrode and electrode drain contacts 31, 33 are bonded to strip conductors 26, 27 respectively, by electrical interconnects, here conventional wires 34, 35 as shown. The outer upper surface 36 of the outer arms of the lower portion 14 have affixed thereto a ceramic or dielectric slab 40. Sandwiched between the dielectric slab 40 and a second dielectric slab 42 are beam lead conductors 44, 46 which pass through the right and left sidewalls of the bottom portion 12 of the package 10. Layer 55 of gold is plated onto the upper portion 54 of dielectric slab 42. The inner ends of the beam lead conductors 44, 46 are wire bonded to the strip conductors 26, 27 by wires 45, 47 as shown. Thus, the bottom portion 12 of the package 10 is formed as an integral structure made up of the base 14 and the ceramic sidewalls formed by ceramic slabs 40, 42. It is noted that the bottom portion 12 of the package has the ceramic slabs 40, 42 and base 14 bonded together to form a hermetically sealed unit prior to the bonding of the transistor 28 to the strip conductors 26, 27 and the top portion 28. That is the bottom portion 12 of the package is formed using high temperature processing and this high temperature processing would destroy the effectiveness of transistor if it were subjected to such temperatures. Thus, the bottom portion 12 of the package is formed as a unit before the transistor electrodes are bonded as described. Once the bottom portion 12 of the package 10 is formed with the beam lead conductors 44, 46 wire bonded to strip conductors 26, 27 and the field effect transistor 28 wire bonded to strip conductors 26, 27 a ceramic cover 50 having a gold rim 52 bonded to the outer periphery of the ceramic cover 50 is placed on and bonded to the bottom portion 12 of the package 10. More particularly, the upper surface of the ceramic slab 42 is soldered to the gold rim 52 via a layer 54 of solder which is disposed on gold plated layer 55 to thereby hermetically seal the package 10. The use of the solder layer 54 to hermetically seal the gold rim 52 to the gold plated layer 54 allows for the hermteic seal to be performed at a relatively low temperature, such temperature being low enough so that the transistor 28 is not damaged as a result of the hermetic sealing of the top 50 and the bottom 12 after the transistor 28 is bonded to the portion 30 and microstrip transmission lines 18, 20. While such package 10 serves adequately in many microwave applications in other applications such package provides only a limited operating bandwidth.